Postcodes in UK
The postcodes in UK are alphanumeric and between five and eight characters long (including a single space separating the outward and inward parts of the code), e.g. the code for the House of Commons is SW1A 0AA. These codes were introduced by the Royal Mail between 1959 and 1974. Once postcodes were introduced, other applications became possible.
Although postcodes are usually assigned to geographical areas, special codes are sometimes assigned to individual addresses or to institutions that receive large volumes of mail, such as government agencies and large commercial companies, e.g. the code for the House of Commons is SW1A 0AA. -- see Speical Postcodes
The 'Outward' part of the postcode denotes the postal district - for example RH for the Redhill area, and then the following number distinguishes the post town - broadly speaking the Delivery Office which services the local area. So RH1 is Redhill itself, RH10 is Crawley. With larger towns there may be more than one number in the outward section - Crawley includes RH10 and RH11. The reverse situation is uncommon but can also occur, with a single postal district lying within more than one post town. The 'Inward' part denotes particular parts of the town / Delivery Office area, with the first part - the number - being a sector, and the final two letters denoting a property or group of properties within that area. In the case of a large office block, for example, the 'Inward' part of the code may denote just a part of the office block, or often just a single company within that block (particularly where the company receives a large amount of mail). In some cases (for instance DVLA) the "inward" code may serve to direct mail to different parts of the same organisation.
A series of five-digit codes may also be used on business mail. This is called Mailsort – but is only available for mailings of 'a minimum of 4,000 letter-sized items'. Discounts are available for such bulk mailings based on the type of mail and how pre-sorted it is.